1. Field of the Invention
The present invention relates to a true/false determining apparatus, an image forming apparatus, a true/false determining method, and an image forming method that ascertain the genuineness of a material, for example, whether or not a printed material such as paper is the original one. More particularly, the present invention relates to a true/false determining apparatus, an image forming apparatus, a true/false determining method, and an image forming method having a mechanism to recognize the intrinsic property of a surface of a material using coherent light.
2. Description of the Related Art
A material such as paper has roughness on its surface that is impossible to be modified controllably. There have been disclosed methods of ascertaining the genuineness of a material using the roughness.
For example, a method has been proposed by which weaving patterns of fiber in a predetermined area on the surface of paper are optically read and registered with a server as a reference image. Then, at true/false determination, the surface state of the above predetermined area or a wide range of area including the predetermined area on the object is optically read and the true/false determination is performed by carrying out pattern matching between the surface state and the registered reference image (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 2005-03839).
In the above conventional true/false determining method, the true/false determination is performed by binarizing image data of the optically read surface and carrying out the pattern matching on the binarized image data. However, paper fiber is so fine that the S/N ratio could not be achieved that is high enough to be used for the determination even if the image data of an optically read surface is binarized. Furthermore, in the case of relatively smooth-surfaced paper such as coating paper or image-formed paper, it is difficult to recognize its surface pattern, therefore the recognition rate of its surface pattern is low.
In addition, one of the methods of detecting roughness of a surface of a material uses the speckle phenomenon. In a speckle phenomenon, when coherent light such as laser is radiated onto a surface of a material, a random-dot pattern emerges on an observed surface exposed to the reflected light. Coherent light is, for example, single-wavelength and single-phase light with high coherency.
A speckle pattern, which emerges as coherent light beams scattered due to roughness of a surface of a material interfere with each other on an observed surface, has a pattern specific to the surface of the material. It has an advantage of raising the S/N ratio relative to the case that the structure on a surface of a material is observed using incoherent light, instead of coherent light.
A method of identifying a material using a speckle pattern is, for example, disclosed in “‘Fingerprinting’ documents and packaging”, Russel Cowburn, Nature, vol. 436. In this document, a method is proposed by which a speckle pattern on a surface of an original document is previously registered as reference image data, and then the true/false determination is performed by carrying out pattern matching between the reference image data and the speckle pattern of the surface of the object of the determination.
However, since the conventional true/false determining method according to “‘Fingerprinting’ documents and packaging”, Russel Cowburn, Nature, vol. 436 uses, for example, a source of single-wavelength laser light, a speckle pattern cannot be achieved if there is an area to absorb said wavelength on paper. In this case, the true/false determination is impossible.
For example, in the case of the true/false determination using a speckle pattern and semiconductor laser with wavelength of 440 nm, if there is a carbon black marking area in the vicinity of an extreme edge of paper, a speckle pattern of scattered light intensity distribution is achieved as shown in FIG. 7. In an image part, scattered light (reflected light) is not achieved because laser light is absorbed there. Because of this, no speckle pattern is achieved corresponding to the image part. On relatively smooth-surfaced coating paper, for example, high-rising is recognized with respect to scattered light intensity distribution of a speckle pattern corresponding to an area on which no image is formed, as shown in FIG. 8.
In the above situation, if binarization is performed on a speckle pattern on a surface of a material detected by including an area with no speckle pattern, for example, binarized patterns representing an image area and an non-image area are detected, resulting no precise binarized pattern. That is, there is possibility that sufficient S/N ratio cannot be achieved by the conventional true/false determining method using a speckle pattern.
In addition, there is a possibility that the frequency of a speckle pattern is high to be used for authentication, so that it takes a long time to perform the pattern matching on a wide area to be authenticated.